Methods for drilling with casing have been developed wherein drilling tools are retrievable through the casing, which acts as the drill string. Much research has also been dedicated to exploring ways to develop a liner drilling technology based on the conventional method of running a liner on drillpipe. There are significant technical and economic hurdles to overcome to develop a retrievable liner drilling system using drillpipe as the means of driving the liner. However, a liner drilling system based on using the already developed Casing Drilling tools® would be much more economical to develop and provide to the market.
There is a relatively large usage of liners in drilling programs, particularly those in lateral wellbores and for deepwater wells where the pressure gradient is very steep. In fact, all casing strings in deepwater wells must be run as a “liner” because the top of the casing is located at the sea bed which may be several thousand feet below the rig floor.
A prior art assembly for liner drilling is shown schematically in FIG. 1. Prior art liner drilling systems normally utilize a drillable bit 10 attached to the bottom of the liner 12 and a liner hanger 14 to attach the top of the liner to drillpipe 16. The drillpipe then extends back to the surface 17. This assembly is used to drill a wellbore 18 to accommodate the liner. Drilling fluid circulation and drillstring rotation is provided to the liner through the drillpipe. Once the liner is drilled to the desired depth, for example to a point where the liner has penetrated the formation, the liner hanger is activated to support the weight of the liner in the wellbore and the drill pipe is detached from the liner. In some cases a liner hanger is not used and the top of the liner is left unsupported. In other cases the top of the liner is expanded to engage the wellbore open hole or liner in which it is installed. The liner may be cemented in place before the drillpipe is detached from the liner hanger. Once the drillpipe is withdrawn from the well, a new drilling assembly may be made-up to the drillpipe and run into the well to drill through the “drillable” bit that remains attached to the liner.
There are a number of disadvantages of this method of liner drilling. First, there is no convenient way to replace the drill bit in cases where it wears out before reaching the desired liner setting depth. Replacing the bit requires the entire liner and drillpipe string be tripped out of the well and this defeats the advantages of drilling with the liner. There is no ability to drill directional wells with the system because it is not possible, to recover the expensive directional drilling and formation logging tools that are required to drill directional wells. The drilled hole must be no larger than the ID of the previously set casing because the “drillable bit” is of a fixed diameter. This may induce undesirable high circulating fluid pressure losses in the annulus.